Electrochemical cells, commonly known as “batteries,” are used to power a wide variety of devices used in everyday life. For example, devices such as radios, toys, cameras, flashlights, and hearing aids all ordinarily rely on one or more electrochemical cells to operate.
Electrochemical cells, such as metal-air electrochemical cells commonly utilized in hearing aids, produce electricity by electrochemically coupling in a cell a reactive gelled metallic anode, such as a zinc-containing gelled anode, to an air cathode through a suitable electrolyte, such as potassium hydroxide. As is known in the art, an air cathode is generally a sheet-like member having opposite surfaces that are exposed to the atmosphere and to an aqueous electrolyte of the cell, respectively. During operation of the cell, oxygen from the air dissociates at the cathode while metal (generally zinc) of the anode oxidizes, thereby providing a usable electric current flow through the external circuit between the anode and the cathode.
Many metallic-based gelled anodes are thermodynamically unstable in an aqueous neutral or alkaline electrolyte and can react with the electrolyte to corrode or oxidize the metal and generate hydrogen gas. This corrosive self-discharge side reaction can reduce both service and shelf life of electrochemical cells that use zinc as the anodic fuel. During discharge, electrochemical oxidation occurs at the anode, and metallic zinc is oxidized to zinc hydroxide, zincate ions, or zinc oxide. Under conditions such as high discharge rates or low electrolyte concentration, where the product of discharge is too densely attached to the surface, passivation of the zinc can occur. The presence of a solid phase zinc oxide or hydroxide film can interfere with the discharge efficiency of the zinc-based anode.
To combat these problems, mercury has conventionally been added to the zinc-based anode to improve the corrosion resistance and discharge behavior of the anode. Additionally, technologies aimed at substituting other components for mercury have been developed. With these technologies, small amounts of lead, calcium, indium, bismuth, and combinations thereof have been combined with zinc to provide a zinc alloy. Unfortunately, it has been shown that many of these alternative materials (i.e., mercury-free) tend to exhibit a drop in both operating voltage and service life as compared to zinc anodes containing a mercury additive. These limitations may be especially noticeable when the cell is discharged at a high rate. This is most likely due to either zinc particle surface passivation, caused by zinc oxide forming at the zinc surface, and/or anode polarization. These may both be caused by the lack of a sufficient quantity of hydroxyl ions in the anode, and/or a sufficiently even distribution of hydroxyl ions.
To improve the performance of an electrochemical cell in the absence of mercury in the anode, it has been suggested in U.S. Patent Application No. 2002/0177043A1 to introduce an ionically conductive clay additive into the zinc-containing anode. The ionically conductive clay material improves the transport of hydroxyl ions inside the zinc anode matrix during a discharge resulting in increased electrochemical cell performance. One suitable ionically conductive clay additive is the synthetic clay Laponite® (Na0.700.7+[(Si8Mg5.5Li0.3)O20(OH)4]0.7−). Although ionically conductive clay additives such as Laponite® in the zinc-containing anode have generally improved performance, there continues to be a need to improve performance of the electrochemical cell over long periods of time.
As such, it would be desirable to provide an electrochemical cell comprising an additive that can be used with or without mercury addition to extend the service life and performance of the electrochemical cell. It would also be desirable for the additive and electrochemical cell to be stable over long periods of time to improve shelf life and performance.